This invention relates to the production of liquid droplets and solid particles having very narrow size distributions. More particularly, this invention relates to methods and apparatus for the production of metal oxide and metal oxide precursor particles having a generally spherical shape and very narrow size distribution.
The uses for metal oxide powders are numerous and varied, ranging from active ingredients in antiperspirants to ceramic raw materials. In the vast majority of these applications, the size distribution and shape of the particles which make up the powder are of critical importance. In the processing of ceramic articles, for example, dense particles having a generally spherical shape and narrow size distribution pack into a highly dense green body, which allows sintering at low temperatures and provides greater strength and density in the final ceramic body. In other applications, it is desirable to prepare multicomponent metal oxide particles which not only have the above characteristics, but also are chemically homogeneous. For example, particles of this type are advantageous in the production of structural ceramics, wear resistant ceramics, electronic dielectric materials, electronic substrate materials, phosphors, and potentially as precursors for optical waveguide preforms.
One known method for the synthesis of multicomponent metal oxide powders involves the evaporative decomposition of solutions. In the practice of this method, a dilute salt solution is atomized to form a spray of droplets, after which solvent is removed from the droplets by the application of heat to form dry particles of the solute. These dry particles are generally decomposable, through the further application of heat during calcining, to the metal oxide itself. Many other methods for producing metal oxide particles also require the production of a spray of droplets.
In most of these methods, the liquid droplet production is critical not only to the shape and characteristics of the final solid particles, but also to the yield of the particles from the process. One method which is typically used to produce a droplet spray is known as a "double fluid atomization". In this method, the liquid to be atomized is forced from a jet nozzle to form a thin liquid stream which is contacted by a second stream of high pressure gas resulting in a highly turbulent mixing and break up of the liquid stream into minute droplets. See for example the report by D. L. Chess, et al in the article "Precursor Powders for Sulfide Ceramics Prepared by Evaporative Decomposition of Solutions", Communications of the American Ceramic Society, November, 1983. One disadvantage of this method is that the liquid droplets which are produced have a very wide size distribution, which in turn results in particles having a very wide size distribution.
An alternative method is known as the vibrating orifice method or the Rayleigh instability method. This method generally produces droplets having a very narrow size distribution and is based upon the principle that a thin liquid stream emitted from a opening under pressure is by nature unstable and will soon disintegrate into droplets by any external forces acting thereupon. The collapse of such a stream into uniform droplets is attained by applying to the stream a periodic vibration of the appropriate amplitude and frequency. It should be noted that the term "vibrating orifice" is actually a misnomer since the orifice itself need not vibrate; disintegration of the liquid stream occurs in the same manner irrespective of where or how the oscillatory vibrations are imparted to the liquid. One characteristic of the vibrating orifice method is that the size distribution of the resulting droplets will depend mainly upon the diameter of the thin liquid stream, which itself depends on the orifice diameter, and the frequency of the vibrations. One prior art application of the Rayleigh instability technique has been to use long capillary tubes to produce the thin liquid streams required by that method. See for example U.S. Pat. No. 3,352,950--Helton et al. While the use of capillary tubes according to this technique may be advantageous for certain purposes, it is not practical for the production of micron size droplets on a commercial scale. For example, if the inner diameter of the capillary is greater than about 30 microns, then highly diluted solutions would be required to obtain solid particles having diameters below about 3 microns. As a result only a very small yield of solid particles would be obtained. On the other hand, if the inner diameter of the capillary is significantly less than about 20 microns, then the liquid pressure required to form a thin stream according to the Rayleigh instability method becomes extremely large. More specifically, a capillary tube having a length of approximately one centimeter and an inner diameter of approximately 5 microns would require a liquid pressure of over 100,000 psi to form a suitable liquid stream. Providing equipment to operate under such conditions is clearly undesirable.
In other applications a thin plate or foil having an orifice therein is used in place of the capillary tube to produce the thin liquid stream required by the Rayleigh instability method. Prior art apparatus and methods have generally been limited to the use of plates having a single orifice. Several advantages result from the use of the such single orifice nozzles. For example, the use of a single orifice eliminates one possible source of variation in droplet size and thus results in a more easily obtained uniform particle distribution. Another advantage is that the single orifice has a minimal impact on the mechanical stability of the orifice plate. On the other hand, however, the production rate of such single orifice generators is limited to the flow rate of the single liquid stream. Accordingly, many generators would be needed to produce large quantities of droplets. Due to the high capital cost associated with each generator, this is clearly not a desirable option. As a result, the prior art vibrating orifice methods have suffered the disadvantages of the single orifice generator in order to achieve uniform droplets. The present invention provides a method and apparatus having the advantages of the single orifice generator without its disadvantages.